1. Field of the Invention
The present invention relates to a pyroelectric thermal radiation sensor and more particularly to a single ended pyroelectric thermal radiation sensor having two elements connected in series opposing fashion having means for preventing static charge build up.
2. Description of the Prior Art
The use of pyroelectric sensors using polarized crystals and plastic films for detection of thermal radiation is well known. There have been many uses developed for such pyroelectric sensors. One important application is in the field of intruder alarms in which a pyroelectric sensor is arranged to detect the infrared radiation from a human being as an indication of an intruder. However, some prior art intruder alarms can be falsely triggered by other sources of infrared radiation such as hot air from a heating system, sunlight, or equipment which may give off heat radiation. To discriminate between an actual intruder and such environmental effects, it is known to require movement of the source of heat. For this purpose, it is common to utilize at least two collocated detectors in a differential connection so as to produce zero net signal output when both detectors are irradiated and to produce multiple pulse outputs when a person walks past the device. A typical example is shown in U.S. Pat. No. 3,839,640 to Rossin which teaches the use of two pyroelectric sensors formed from a single piece of polyvinylidene fluoride (PVF.sub.2) plastic film. This patent shows two such sensors connected in series opposition. The Rossin detector, which uses a single PVF.sub.2 film, has a "floating" rear electrode and two front electrodes such that a series opposition connection is obtained with respect to the two front electrodes. The detector drives a single-ended amplifier. Although there is no external connection to the rear electrode, PVF.sub.2 has a finite leakage resistance which serves to bleed off any static charge build up on the "floating" electrode.
Pyroelectric crystals have been found to be much superior to pyroelectric PVF.sub.2 films since the plastic film is flimsy and easily damaged. The output for a given area PVF.sub.2 sensor is much less than for crystal types. Liu, in U.S. Pat. No. 3,816,750, discloses a dual detector, series opposition connected differential sensor utilizing a polarized crystal which produces a higher output than a similar size PVF.sub.2 type. A problem can arise with the Liu type sensor if used to drive a single ended amplifier with the common electrode floating. As is well known, the resistivity of the inorganic pyroelectric crystals is very high compared to PVF.sub.2. When the common electrode is left floating, electrostatic charges can build up on one or the other element. Experiments have shown voltages in the 100-300 volt range can occur. The crystal may attract small dust particles or other contaminates which impinge causing momentary changes in dielectric constant. These actions can cause random noise and biasing problems when directly coupled to very high impedance amplifiers such as field effect transistors (FET).